1. Field of the Invention
Embodiments relate to semiconductor memory devices. More particularly, embodiments relate to a programming method for a nonvolatile semiconductor memory device.
2. Description of the Related Art
Semiconductor memory devices may be classified into random access memories (RAMs) and read-only memories (ROMs). ROMs retain data even without power. There are various types of ROMs, e.g., programmable ROMs, erasable and programmable ROMs, electrically programmable and erasable ROMs, flash memories, and so on. Flash memories may be further differentiated in NOR and NAND types. NAND-type flash memory devices are now widely employed in memory cards capable of storing large-quantities of data, because they allow higher integration density than NOR-type flash memory devices.
For the purpose of extending data storage capacity of flash memory devices, many memory cell architectures capable of storing multiple bits have been examined. A memory cell able to store multiple bits is called a ‘multi-level cell’ (MLC). A memory cell only able to store one bit is called a ‘single-level cell’ (SLC). Since MLCs each store m-bit data (m is an integer equal to or greater than 2), threshold voltage distributions for MLCs should be more precisely controlled than for SLCs.
In a MLC flash memory device, an incremental step pulse programming (ISPP) scheme is used for precisely controlling the threshold-voltage distributions. According to the ISPP scheme, threshold voltages of the memory cells increase in proportion to a voltage increment of a programming voltage. Thus, by repeating programming loops, the programming voltage applied to a word line increases stepwise to elevate the threshold voltages of the memory cells.
Each programming loop includes of a programming period and a program-verifying period. The programming voltage increases by a predetermined voltage increment (ΔV) by the repetition of the programming loop. The threshold voltage of the memory cell being programmed increases in proportion to the voltage increment (ΔV).